Recently there are movements for introducing biodiesel fuels (BDF) produced using, as a starting material, oils derived from vegetable oils such as palm oil from the viewpoint of prevention of global warming and circulation of resources. Also, from the viewpoint of protection of environment, trials to use, as fuels for diesel engine, fuels prepared using waste food oils after the use for cooking and trap grease (sludge-like oils and fats floating on a surface of waste water) have been made.
Biodiesel fuels are prepared by ester exchange reaction of monoglyceride, diglyceride and triglyceride, which are main components of oils and fats, with a lower alkyl alcohol such as methanol and ethanol. For example, an alkali catalyst method, in which oils and fats are subjected to ester exchange with a lower alkyl alcohol such as methanol in the presence of an alkaline catalyst such as NaOH or KOH, has been proposed (for example, cf. Patent Document 1)). In such a method, glycerin amounting to about 10% to 20% of starting oils and fats is generated as a by-product.
However, in this method, since the ester exchange reaction is carried out for about one day with stirring under heating, there is a problem that the production cost thereof becomes high. In addition, because an alkaline catalyst is used, a reaction proceeds between glycerin and the alkaline catalyst and soap is generated. If soap is generated, it takes time, i.e., about one day to separate a biodiesel fuel from glycerin. Further, once soap is generated, it is necessary to waste glycerin and soap which are intrinsically valuable as a commercial product. As a result, a new environmental problem arises.
In order to solve such a problem, the inventors of the present invention have proposed the method of separating a biodiesel fuel from glycerin by carrying out an ester exchange reaction with stirring using ultrasonic waves (for example, cf. Patent Document 2)). This method is characterized in that the reaction time is as short as five hours as compared with conventional methods, the reaction advances even with a small amount of a catalyst to be added, there is no need of applying heat in the ester exchange reaction, and soap is hardly generated.
However, in this method, equipment for stirring and separating with ultrasonic waves is necessary. Also, there is a problem that for stirring and separating with ultrasonic waves, higher production cost is required. Further, the separation time is as short as one hour as compared with conventional methods. However, a problem with generation of soap during the one-hour separation remains unsolved. Therefore, there is a problem that high quality glycerin cannot be obtained.
In order to solve such a problem, trials for adding a solvent to a reaction system and carrying out the reaction in a homogeneous phase system have been made (for example, cf. Patent Document 3)). In the method described in this document, dimethyl ether is added to a reaction system, thereby greatly improving the speed of the ester exchange reaction, and efficiently carrying out the ester exchange.
However, by the use of dimethyl ether, a lower alkyl alcohol as a reactant is easily dissolved in the generated glycerin, thereby causing a problem that the proceeding of the reaction is hindered, i.e., yield of a fatty acid alkyl ester becomes low. Also, since the lower alkyl alcohol is dissolved in either of the fatty acid alkyl ester phase and the glycerin phase, there is a problem that it takes a time to separate the fatty acid alkyl ester phase containing the fatty acid alkyl ester, lower alkyl alcohol and organic solvent from the glycerin phase. If it takes time to separate the fatty acid alkyl ester phase from the glycerin phase, soap is generated, resulting in a problem with a production cost. Also, because an unreacted lower alkyl alcohol is dissolved in the glycerin phase, it is necessary to extract the unreacted lower alkyl alcohol from the recovered glycerin. Dimethyl ether is an organic solvent which is not dissolved in water. Therefore, if water is contained in the reaction system, there is a problem that the reaction does not advance. Further, a boiling point of dimethyl ether is as very low as −23.6° C. Therefore, in order to let dimethyl ether remain in the reaction system, it is necessary to use special equipment such as a pressure cell.
Patent Document 1: JP-A-7-197047
Patent Document 2: JP-A-2004-156022
Patent Document 3: JP-A-2009-40979